Perhaps you just mean to convert it from "summation form" ("sigma form") to a written out form?

For something like #\sum_{i=1}^{n}i^{2}#, the summation symbol #\Sigma# just means to "add up". Putting an #i=1# underneath the summation symbol means to start the value of #i# at 1. It is then assumed that #i# keeps increasing by 1 until it reaches #i=n#, where #n# is the number above the summation symbol. The #i^2# represents the formula for the terms that get added, first when #i=1#, then #i=2#, then #i=3#, etc..., until #i=n#.

Therefore, the answer would be #\sum_{i=1}^{n}i^{2}=1^{2}+2^{2}+3^{2}+\cdots+(n-1)^2+n^2#.

This example is interesting in that there is a shortcut formula for adding up the first #n# squares: it equals

#\frac{n(n+1)(2n+1)}{6}=\frac{1}{3}n^[3}+\frac{1}{2}n^{2}+\frac{1}{6}n.#

You should take the time to check that this works when, for instance, #n=5#.